The present invention relates to a Micro-Electronic-Mechanical-System (MEMS) scan controller and a control method thereof, especially to a controller for micro-electric-mechanical mirrors (MEMS mirror) applied to bi-direction laser scanning units (LSU) and a control method thereof that perform scanning by the MEMS mirror with inherent resonant frequency within time interval of emittance of laser source.
Most of LSU available now uses a polygonal mirror rotating at high speed to control reflection direction of laser beam. However, due to hydraulic driving, working rotational speed limits, high manufacturing cost, high noises and delayed initiation, such LSU is unable to meet requirements of high speed and high precision by using polygon mirror. In recent years, MEMS mirrors with torsion oscillators are getting known and are going to be applied to LSU of imaging systems, scanners or laser printers in future. The MEMS oscillatory mirror developed based on principle of torsion oscillators has higher scanning efficiency than conventional polygon mirror.
In a laser scanning unit (LSU), a Micro-Electronic-Mechanical-System (MEMS) oscillating mirror mainly consists of a torsion oscillator and a mirror. The MEMS mirror is controlled by a MEMS scan controller. MEMS mirror driven by resonance magnetic field symmetrically oscillates along an axis. When a laser light is emitted to the mirror of the MEMS mirror, the MEMS oscillating mirror reflects the incident laser beam to the axis of the MEMS mirror at different angles for scanning along with different reflecting angles of the mirror surface that changes with time. Since the MEMS mirror scanning approach can neglect the wavelength effects, that the MEMS mirror has features of high resolution and large rotation angle so that has been applied broadly to commercial products, science and industries, such as devices disclosed in U.S. Pat. Nos. 5,408,352, 5,867,297, 6,947,189, 7,190,499, US App. No 2007/0063134, TW Patent M253133, TW 1283952 and JP 2006-201350, JP63-314965, etc. In order to improve scanning efficiency, a bi-directional LSU is developed yet associated control difficulties are raised.
Due to resonant oscillation of the MEMS mirror, the rotation angles and stability of the MEMS have effects on precision of the LSU. In a controller for bi-directional LSU of the MEMS mirror, conventional technique focuses on stability control of the MEMS mirror such as adjustment of resonant frequency, working angle, or by means of a voltage controlled oscillator (VCO) to control the frequency. The frequency control of the VOC is based on changing permittivity of dielectric material by current or change of the capacitance by the voltage, as shown in US2005/0280879, US2006/0139113, US2005/0139678, US2007/0041068, US2004/0119002, U.S. Pat. Nos. 7,304,411, 5,121,138, and JP63-314965, etc. Take a bi-directional LSU with 600 dots-per-inch (dpi) resolution per A4 size as an example, 5102 light spots are sent per each scanning in one directional. The 5102 light spots are sent completely during an imaging interval while the imaging interval should be invariant with the frequency or amplitude variations of the MEMS mirror that lead to deviation of the light spot and the image is not formed on the object. Thus the calculation frequency of the MEMS mirror for sending correct signal to the laser controller that emits laser light is a main point of control.
Various MEMS mirrors have a bit difference in their structure and thus their inherent resonant frequency are different. As show in US2006/0279364, U.S. Pat. Nos. 6,891,572, 6,870,560, 6,987,595, 6,838,661, and US2006/0117854, disclosed the techniques as the MEMS mirror are controlled by reference table, synchronous scan signal, counter or detection of PD position, etc. A control way revealed by the GB2378261 is by prediction molding. In JP2226114, a comparison way of dual voltage is used. However, these control devices have complicated structure, sophisticated calculation and large memory so as to control stability of the MEMS mirror oscillation and make oscillation of the MEMS mirror with different resonant frequency become consistent. Therefore, the MEMS LSU can scan precisely within the effective scanning window.